1. Field of the Invention
The present invention relates to apparatus and processes for graphics processing systems. More particularly, it relates to apparatus and methods for testing objects displayed on a graphics device to establish the relative positions and interactions of those objects.
2. Background and Related Art
Graphics display systems are used to provide computer system users with information and data in a readily understood form. Many graphics systems display data from different sources or applications in windows on the display screen. Windows are used to implement a "desktop" metaphor allowing the system user to select a window and place it on "top" of the visual desktop making it the active window. Other windows may be entirely or partially obscured by the active window.
The graphics display system responsible for drawing the windowed image must determine what, if any, data from a window is to be displayed. The system must determine the relative interaction between windows, i.e. whether one window is contained within another, whether the windows are disjoint, or whether some part of the windows overlap. The graphics system could spend considerable processing time drawing to an obscured window.
Graphics systems ideally determine the whether a window is obscured by another window before performing window drawing activities. This can, however, cause a performance penalty due to the need to test the four vertices of the first window against each of the four vertices of the second window--a total of sixteen comparisons. The Sutherland-Hodgman polygon clipping algorithm and Cohen-Sutherland line clipping algorithm are of this type and are described in Computer Graphics, Principles and Practice, J. Foley, A. van Dam, S. K. Feiner and J. F. Hughes, Addison-Wesley, 1990.
A system and method for quickly determining the relative interaction of rectangles (most windows are rectangular) is needed to increase the performance of graphics systems.